The most commonly used indium compound for MOCVD or other epitaxial deposition processes is trimethylindium. Materials produced by epitaxial growth using trimethylindium as an indium source include, for example InP, InGaAs, InGaAlP, InGaAsP, InGaAs/GaAs/AlGaAs, InAs, InSb and InAsBi.
A well-known inherent disadvantage of trimethylindium as an indium source is the fact that it is a solid at room temperature. Liquid sources of organometallic vapors are preferred to solid sources because a gas stream having a substantially constant partial pressure of organometallic vapors can be produced merely by bubbling carrier gas through the liquid at a constant rate. With solids, on the other hand, the surface area is constantly changing as the organometallic source is vaporized. Furthermore, solids, such as trimethylindium, tend to recondense on surfaces of the gas pathway, further increasing the difficulty of providing a uniform partial pressure of the organometallic compound.
Trimethylindium, being a solid, is an anomaly relative to other trialkylindiums, as C.sub.2 -C.sub.5 alkylindiums are liquids at room temperature. This anomaly is the result of trimethylindium existing as a tetramer at room temperature, whereas other trialkylindiums are monomers. However, from the standpoint of providing a sufficiently high vapor pressure for epitaxial growth applications, trimethylindium, having a relatively high vapor pressure, tends to be preferred.
Several approaches have been taken to eliminate complications due to uneven mass flow when using trimethylindium. Some of these include reverse flow of carrier gas through the bubbler; packing the trimethylindium in inert material, such as Teflon beads, in the bubbler; use of two or more trimethylindium bubblers in series; and "solution trimethylindium" where the trimethylindium is dissolved and/or suspended in a high boiling amine or high boiling hydrocarbon.
Those approaches in which the trimethylindium remains in solid phase, reduce, but do not eliminate, uneven mass flow.
While trimethylindium may be dissolved in amine, its solubility is low, typically about 20%, requiring a large volume of trimethylindium source. Amines complex with trimethylindium, advantageously breaking up the tetramer, but disadvantageously tying up trimethylindium. Because epitaxial growth applications require a source with very minimal high vapor pressure impurities, amine solvents are used which are highly purified, particularly with respect to volatile impurities. Nevertheless, if impurities are present, they may react with the trimethylindium, producing new, more volatile impurities. Also, even high boiling amines are entrained in the gas stream to some extent and may undesirably introduce nitrogen into the material which is being produced.
High boiling hydrocarbons avoid the problem of nitrogen. However, trimethylindium is even less soluble in hydrocarbons than amines, and the trimethylindium is more dispersed than dissolved in hydrocarbon media. Because hydrocarbons do not break up the tetramer, problems with deposition of trimethylindium in the gas pathway remain. Also, as with amines, there is the possibility that impurities will react with trimethylindium to produce volatile impurities.
U.S. Pat. No. 4,720,560 approaches the problem by mixing two moles of trimethylindium with a mole of triethylindium to produce ethyldimethylindium by the reversible reaction:
2 Me.sub.3 In+Et.sub.3 In.fwdarw.3 EtMe.sub.2 In.
The desired indium source in this approach is not trimethylindium, but ethyldimethylindium. Because the equilibrium is temperature-dependent with the equilibrium shifting to the right at lower temperatures, it is advised to maintain the mixture at a temperature below room temperature, e.g., at about 10.degree. C. Such a lowered temperature may be disadvantageous if high vapor pressures are required for the crystal growth. Increasing the temperature may potentially cause the above equilibrium to shift to the left, and cause the ethyldimethylindium to dispreportionate.